This invention relates to stabilized photopolymerizable compositions based on photopolymerizable ethylenically unsaturated compounds containing benzoin ether initiators. More particularly, this invention relates to the use of certain nitrogen-containing aromatic compounds as stabilizers for photopolymerizable compositions containing photoinitiators.
Photopolymerizable compositions containing unsaturated materials and photoinitiating compounds are well known in the art, being particularly useful for moldings and coatings. These materials polymerize at low temperatures when exposed to actinic radiation. While numerous compounds have been found useful as photoinitiators for the polymerization of unsaturated compounds, the ethers of benzoin have found wide-spread commerical acceptance. These benzoin ethers exhibit excellent cure rates, rendering them particularly attractive where rapid hardening is desirable. The use of benzoin ethers as photopolymerization initiators is described in U.S. Pat. No. 2,448,828.
While the benzoin ethers are widely used because of their excellent curing properties, they are not wholly satisfactory with regard to storage stability. Thus, unsaturated systems to which benzoin ethers are added have considerably diminished dark storage stability and will gell prematurely in many systems where storage is a key factor.
Various attempts have been made to remedy this deficiency of the benzoin compounds by including stabilizing additives in the photopolymerizable composition. For example, U.S. Pat. No. 2,647,080 discloses the addition of allyl glycidyl ether to systems containing halogen-free acrylate and acyloin ether initiators. U.S. Pat. No. 3,814,702 teaches photosensitizing compositions comprising a benzoin ether, an organic acid and a solvent; optionally, a weak reducing agent may be included in the photosensitizing composition. Other multi-component stabilizing systems are described in U.S. Pat. No. 3,819,495, which discloses a copper compound soluble in polyester resin and an organic compound having ionically-bound chlorine or capable of forming chlorine ions in situ as stabilizers for polyester resins containing benzoin ethers, while U.S. Pat. No 3,819,496 discloses similar systems employing an iron and/or manganese compound instead of the copper compound.
Other compounds have been added to benzoin ether systems as cure accelerators. For example, Japan Kokai 73 00, 983 discloses the use of dialkylaminobenzoins in conjunction with benzoin methyl ether as curing agents for polyester resins, while Japan Kokai 73 75,638 teaches mixtures of benzoin alkyl ether and p,p'-bis (dialkylamino) thiobenzophenone as curing agents that provide colored products. In German Offen. 2,263,804, the addition of ascorbic acid or thiourea to benzoin ethyl ether to reduce the exposure time of photopolymer printing reliefs is disclosed. While functioning to accelerate curing, many of these compounds are known to decrease the storage stability of the system.
Now it has been found in accordance with this invention that selected nitrogen-containing aromatic compounds are excellent stabilizing agents for photopolymerizable compositions containing photoinitiators. These stabilizing agents function to improve the dark storage capability of the composition without significantly detracting from the cure rate.